<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        div{
            width: 700px;
            height: 200px;
            background-color: burlywood;
        }

        /* 参考父元素 */
        p{
            width: 50vw;
            height: 50vh;
            background-color: red;
        }
    </style>
</head>
<body>
    <div>
        <p></p>
    </div>
</body>
</html>